Temperature driven hydrogen-induced disproportionation of Zr2Cu

Article abstract of DOI:10.1016/j.jallcom.2009.07.179

The absorption of hydrogen by Zr₂Cu was studied by the aid of the Zr₂Cu-H phase diagram elaborated by Kadel and Weiss. It was found that Zr₂Cu hydride was formed at ambient temperature after moderate hydrogen absorption. H

Full text of DOI:10.1016/j.jallcom.2009.07.179

Journal of Alloys and Compounds 487 (2009) 489–493
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Temperature driven hydrogen-induced disproportionation of Zr2Cu
Masanori Hara∗, Yukiko Hayashi, Kuniaki Watanabe
Hydrogen Isotope Research Center, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 June 2009
Received in revised form 23 July 2009
Accepted 25 July 2009
Available online 11 August 2009
The absorption of hydrogen by Zr2Cu was studied by the aid of the Zr2Cu–H phase diagram elab-
orated by Kadel and Weiss. It was found that Zr2Cu hydride was formed at ambient temperature
after moderate hydrogen absorption. Hydrogen-induced disproportionation of Zr2Cu occurred at tem-
peratures exceeding 550 K. The unknown compound of the ␥ phase reported by Kadel and Weiss
was identified as Zr7Cu10. It was also found that the ␤ phase consisted of ZrCu and Zr hydride
instead of Zr2CuHx, x = 1.0–1.4. The disproportionation behavior was found to follow a different mecha-
nism below and above 973 K. Below 973 K the disproportionation was found to proceed by the reaction
sequence, Zr2Cu + H2 → Zr7Cu10 + ZrHx + H2(␥ phase) → Zr14Cu51 + ZrHx + H2(␦ phase), and above 973 K
via the sequence, Zr2Cu + H2 → ZrCu + ZrHx + H2(␤ phase) → Zr7Cu10 + ZrHx + H2(␦ phase). The difference
could be explained by the thermodynamic stability of ZrCu intermetallic compounds.
Keywords:
Hydrogen-induced disproportionation
Metal hydride
Zr alloy
Zr2Cu
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
poses below 973 K by an eutectoid reaction [3]. On the other hand,
the disproportionation of Zr2Cu at temperature below 973 K occurs
Zr alloys exothermically react with hydrogen to yield hydrides,
whose potential applications are hydrogen storage, hydrogen
purification, vacuum pumps, catalysts, etc. However, some of Zr
alloys show a tendency to decompose via hydrogen-induced dis-
proportionation [1,2]. Concerning disproportionation, a binary AB
alloy decomposes into an A-hydride and a new metallic compound
deficient in A, where A is the hydride-forming element. The ease of
disproportionation significantly differs from one alloy to another,
and the disproportionation behavior is affected by the existence
of intermetallic compounds deficient in A in comparison with the
original compound.
via following reactions:
Zr2Cu(Hx, x = 0–0.36)(˛ phase)
→ ı-Zr hydride + unknown compound(ꢀ phase)
→ ε-Zr hydride + Zr14Cu51(ı phase).
According to the currently favored Zr–Cu phase diagram, the
intermetallic compounds Zr8Cu5 and ZrCu show eutectoid tem-
peratures of 985 and 1003 K, respectively [4]. These temperatures
are close to the eutectoid temperature of Zr2CuHx, x = 1.0–1.4 [3]. It is
known that ZrCu and Zr8Cu5 can affect the stability of the ␤ phase
of the Zr2Cu–H system.
To elucidate the role of Zr8Cu5 and ZrCu during the hydrogen
uptake by Zr2Cu and to identify unknown phases of the Zr2Cu–H
system, the hydrogen absorbing and desorbing behavior of Zr2Cu
was investigated and the phase transitions were determined by
X-ray diffraction measurements at ambient temperature.
The phase diagram of the Zr2Cu–H system has been reported in
detail by Kadel and Weiss as shown in Fig. 2 of Ref. [3]. They pointed
out that Zr2Cu disproportionates at temperature above 1023 K in
four consecutive reaction steps, i.e.
Zr2Cu(Hx, x = 0–0.36)(˛ phase) → Zr2CuHx, x = 1.0–14(ˇ phase)
→ ı-Zr hydride + unknown compound(ꢀ phase)
→ ε-Zr hydride + Zr14Cu51(ı phase),
2. Experimental
where ZrCu(Hx) and ZrCuHx represent the solid solution phase
and the metal hydride phase, respectively. However, it is diffi-
cult to observe directly the ␤ phase by X-ray diffractometry under
a hydrogen atmosphere. This is because Zr2CuHx, x = 1.0–1.4 decom-
2.1. Sample preparation
The intermetallic compound Zr2Cu was prepared by argon arc melting of Zr
(purity 99.8%) and Cu (purity 99.9%). The prepared ingot was mechanically crushed
to powder and the powder was sifted to obtain a fraction in a range from 100 to
350 ␮m. To remove strains, the sifted powder was heated at 873 K for 2 h under
vacuum. Subsequently, the Zr2Cu powder was analyzed by X-ray diffraction (XRD)
using Cu K␣ line as shown in Fig. 1(a). The diffraction peaks could be assigned to
Zr2Cu. The lattice parameters (see Fig. 1(a)) are in good agreement with the reported
values for Zr2Cu [5].
∗
Corresponding author. Tel.: +81 76 445 6922; fax: +81 76 445 6931.
E-mail address: masahara@ctg.u-toyama.ac.jp (M. Hara).
0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.07.179

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M. Hara et al. / Journal of Alloys and Compounds 487 (2009) 489–493
Fig. 2. Hydrogen absorption curves by Zr2Cu at different temperatures. Hydrogen
gas was loaded to the reaction tube amounting to [H]/[Zr2Cu] = 5. The composition
[H]/[Zr2Cu] (right ordinate) was evaluated from the change in hydrogen pressure.
3. Results and discussion
3.1. Hydrogen absorption behavior
Fig. 2 shows hydrogen absorption curves of Zr2Cu at differ-
ent temperatures. The ordinates of left and right represent the
hydrogen pressure and the amount of absorbed hydrogen, respec-
tively. At each temperature, the pressure decreased with time and
achieved individual values within 150 s, i.e. equilibrium conditions.
The amount of absorbed hydrogen in the equilibrium conditions
was reduced with temperature increase. The ratio of absorbed
hydrogen to Zr2Cu below 873 K was [H]/[Zr2Cu] ≥ 3, whereas it
was only 1.3 at 1023 K. The absorption at 1023 K showed a simple
smooth curve. It implies that the absorption took place via a sin-
gle step. On the other hand, the hydrogen absorption curves below
973 K showed a shoulder. This suggests hydrogen absorption in two
or more reaction steps.
Fig. 1. X-ray diffraction patterns of (a) as-prepared Zr2Cu and (b) its hydride. The
patterns were taken using Cu K␣ line.
Zr2Cu hydride was prepared from the Zr2Cu powder as follows. Three grams
of Zr2Cu powder were introduced into a reaction tube made of quartz glass. The
tube was attached to a high vacuum system that details of the system have been
described elsewhere [6]. After being evacuated below 1 × 10−5 Pa, the reaction tube
was heated at 873 K for 2 h to activate the Zr2Cu powder and then cooled down
to ambient temperature. Subsequently, a predetermined amount of hydrogen was
introduced into the reaction tube at ambient temperature which reacted with Zr2Cu
to yield the hydride. This procedure was repeated until the formation of Zr2CuH4.4
.
Fig. 1(b) shows the XRD pattern of the prepared Zr2Cu hydride. The obtained pattern
agreed well with the patterns of Zr2CuH4.2 provided in the literatures [7,8]. The
lattice constants were evaluated under the assumption that the crystal structure of
Zr2Cu hydride is monoclinic [8].
The hydrogen absorption behavior of Zr2Cu was considered by
taking account of the phase diagram of Zr2Cu–H system reported
by Kadel and Weiss [3]. According to the phase diagram, hydrogen
absorption could proceed as follows:
2.2. Experimental procedures
(a) below 873 K:
2.2.1. Hydrogen absorption measurements
0.5017 g of Zr2Cu powder were introduced into the reaction tube and evac-
uated to less than 1 × 10−5 Pa with the vacuum system [6]. The powder was
heated at 873 K for 2 h to activate the Zr2Cu. Subsequently, the temperature of
the specimen was changed to a predetermined temperature. Hydrogen was sup-
plied to the reaction tube until a ratio of [H]/[Zr2Cu] = 5. The volume of the reaction
tube and the manometers was 495.9 cm3. The hydrogen pressure in the tube was
measured to determine the extent of the hydrogen absorption by Zr2Cu. After
the system achieved equilibrium condition, surplus hydrogen gas was rapidly
pumped off, and the specimen was rapidly cooled down to ambient tempera-
ture.
Zr2Cu(Hx, x = 0–0.36)(˛ phase)
→ ı-Zr hydride + unknown compound(ꢀ phase)
→ ε-Zr hydride + Zr14Cu51(ı phase),
(b) at 973 K:
(1)
(2)
Zr2Cu(Hx, x = 0–0.36)(˛ phase)
→ Zr2CuHx, x = 1.0–14(ˇ phase) → ı-Zr hydride
+ unknown compound(ꢀ phase),
2.2.2. Hydrogen desorption measurements
In a typical desorption experiment, 0.1998 g of Zr2CuH4.4 were placed into
the reaction tube and evacuated to less than 1 × 10−5 Pa at ambient temper-
ature by using the vacuum system [6]. Under these conditions, Zr2CuH4.4 did
not release hydrogen because the surface of the hydride was poisoned by air.
Subsequently, the tube was isolated from the vacuum pumps by the valve. The
specimen was then heated to a given temperature with temperature ramps of
(c) at 1023 K:
Zr2Cu(Hx, x = 0–0.36)(˛ phase) → Zr2CuHx, x = 1.0–14(ˇ phase).
(3)
3.7 or 7.4 K/min, respectively. After attaining
a given temperature, the valve
was opened and the hydrogen released by the specimen was pumped out.
Thereafter the specimen was immediately cooled down to ambient tempera-
ture.
It has been reported that below 973 K the ˇ phase decomposes
owing to a eutectoid reaction [3]. Since the hydrogen absorption of

M. Hara et al. / Journal of Alloys and Compounds 487 (2009) 489–493
491
Fig. 4. X-ray diffraction patterns of Zr2Cu specimens after hydrogen absorption at
873 K. In case of step 1, hydrogen absorption was stopped after 50 s by pumping
the hydrogen and cooling down. The pattern of step 2 is the same as that shown in
Fig. 3(b).
Fig. 3. (a–d) X-ray diffraction patterns of Zr2Cu specimens after hydrogen absorp-
tion at different temperatures. Patterns were taken after hydrogen absorption had
attained the equilibrium conditions (see Fig. 2).
Zr2Cu at temperatures below 973 K passes through some transient
phases, a two-step hydrogen absorption curve can be expected.
To confirm the phase change during hydrogen absorption, spec-
imens were analyzed by XRD. Fig. 3 shows XRD patterns of the
specimens after hydrogen absorption. It is seen that at the exam-
ined temperatures the patterns differ from each other, suggesting
that the absorption of hydrogen gives rise to different reaction
products.
The pattern (a) of Fig. 3 observed after hydrogen absorption at
773 K can be assigned to four compounds, i.e. ZrH1.66 [9], ZrH1.801
[10], Zr14Cu51 [11] and Zr7Cu10 [12]. Phases of ␦- and ␧-Zr hydride
appeared owing to the disproportionation by hydrogen absorption
at 773 K. The amount of absorbed hydrogen yielding Zr hydride at
773 K was not determined from the XRD pattern. On the basis of the
thermodynamic parameters of the Zr–H system reported by Wang
and Olander [13], the composition of the Zr hydride formed at 773 K
was evaluated to be ZrH1.92. Therefore, the hydrogen-induced dis-
proportionation of Zr2Cu at 773 K proceeds according to the overall
reaction:
for 773 K. The evaluated composition of Zr hydride was ZrH1.78 [13].
Consequently, Zr2Cu disproportionates at 873 K in a similar man-
ner as 773 K, whereas the composition of the produced Zr hydride
is not the same.
As apparent from pattern (c) in Fig. 3 at 973 K, the character-
istic peaks of Zr2Cu [5] appear and those of Zr14Cu51 disappear.
The intensity of the Zr7Cu10 peaks [12] was stronger than those
observedattemperaturebelow873 K. On the other hand, Zr hydride
phases were still observed. The composition of Zr hydride was eval-
uated to be [H]/[Zr] = 1.62 [13]. Therefore, Zr2Cu disproportionates
during hydrogen absorption at 973 K by the overall reaction:
10Zr2Cu + 10.53H2 → Zr7Cu10 + 13ZrH1.62
.
(5)
The amount of absorbed hydrogen by reaction (5) is given by
[H]/[Zr2Cu] = 2.1. This agrees well with the amount of absorbed
hydrogen at 973 K shown in Fig. 2. Reaction (5) actually describes
the transformation from the ␣ phase to the ␥ phase as referred to in
scheme (2) shown above [3]. Kadel and Weiss did not identify the
compound in the ␥ phase. On the basis of the results of this work,
51Zr2Cu + 84.48H2 → Zr14Cu51 + 88ZrH1.92
(4)
the unknown compound of the ␥ phase is Zr7Cu10
.
This reaction scheme agrees with the aforementioned scheme
(1). Therefore, reaction (4) describes the transformation from the
␣ phase to the ␦ phase [3]. According to reaction (4), the amount
of absorbed hydrogen at equilibrium should be [H]/[Zr2Cu] = 3.3.
As seen from Fig. 2 the value agrees well with the value of
[H]/[Zr2Cu] = 3.2 observed at 773 K.
Pattern (b) shows that the hydrogen absorption by Zr2Cu at
873 K also generates four compounds, i.e. ZrH1.66 [9], ZrH1.801 [10],
Zr14Cu51 [11] and Zr7Cu10 [12]. With the exception of the relative
peak intensities, this result does not differ much from that obtained
at 773 K (see Fig. 3(a)). The peaks of ZrH1.66 were higher than those
The absorption of hydrogen at temperatures below 973 K pro-
ceeds in two steps as shown in Fig. 2. This is apparent from the
hydrogen absorption curve at 873 K, which shows a shoulder at
about 70 s. To verify this, a hydrogen absorption experiment at
873 K was carried out under somewhat different manner. For this
run, hydrogen absorption was stopped after 50 s by pumping the
hydrogen off and cooling rapidly the specimen to ambient temper-
ature. This run is denominated as step 1, whereas the run at 873 K
shown in Fig. 2 will be referred as step 2. Fig. 4 shows the diffrac-
tion patterns of the specimen after step 1 and step 2. The diffraction
pattern of step 1 consisted of the Zr7Cu10 [12] and Zr hydride [9,10].

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M. Hara et al. / Journal of Alloys and Compounds 487 (2009) 489–493
Accordingly, the reaction at step 1 can be described by
10Zr2Cu + 11.57H2 → Zr7Cu10 + 13ZrH1.78
,
(6)
where the composition of the Zr hydride was evaluated from the
literature [13]. The hydrogen content calculated according to reac-
tion (6) should be [H]/[Zr2Cu] = 2.3. This value is almost the same as
the hydrogen content at the end of step 1 shown in Fig. 2. Reaction
(6) leads to the same products as the disproportionation reaction
at 973 K given by reaction (5). The XRD pattern after step 2, which
was completed in about 200 s to reach [H]/[Zr2Cu] = 3.0, consisted
of ZrH1.78 and Zr14Cu51 [11]. This indicates that step 2 comprises
the change from the ␥ phase to the ␦ phase [3], i.e.
51Zr7Cu10+193.1H2 → 10Zr14Cu51+217ZrH1.78
.
(7)
Therefore, the hydrogen absorption in this temperature range
should proceed the reaction sequence
Zr2Cu + H2 → Zr7Cu10 + ZrHx + H2(ꢀ phase)
→ Zr14Cu51 + ZrHx + H2(ı phase).
(8)
Although Zr2Cu absorbed hydrogen at 1023 K, peaks of Zr2Cu
[5] were still observable in pattern (d) (see Figs. 2 and 3). Only
small peaks which were assigned to Zr7Cu10 [12] and ZrH1.801 [10]
were observed. However, there appeared no diffraction peaks of
Zr2CuHx, x = 1.0–1.4 [3,14], which were reported as the ␤ phase by
Kadel and Weiss [3]. They described that the hydride of Zr2Cu
assigned to the ␤ phase was formed when hydrogen was absorbed
at 1023 K [3]. Using XRD they also identified the hydride phase
when quenching from 1023 K, and no peaks assignable to Zr hydride
was found. In contrast to this, no Zr2Cu hydride peaks could be
observed in the present study (see pattern (d) of Fig. 3). To consider
the hydrogen absorption at temperatures exceeding 973 K, one
should take into account the existence of the ␤ phase reported by
Kadel and Weiss [3]. They found this phase to be stable above 973 K
[3]. According to the currently favored Zr–Cu phase diagram [4], the
intermetallic compound, Zr8Cu5, is stable within the temperature
range 985–1218 K, but it decomposes to Zr7Cu10 and Zr2Cu at tem-
peratures under 985 K owing to the eutectoid reaction. In addition,
ZrCu is also stable in the temperature range 1003–1226 K [4] and
decomposes to Zr7Cu10 and Zr8Cu5 at temperatures below 1003 K
according to the eutectoid reaction. These temperatures are close
to the eutectoid temperature of the ␤ phase [3]. Since the XRD
pattern shown in Fig. 3(d) is consistent with a mixture of Zr2Cu,
Zr7Cu10 and Zr hydride, it is considered that these compounds were
formed by the eutectoid reaction of Zr8Cu5 or ZrCu. Therefore, the
hydrogen-induced disproportionation of Zr2Cu at 1023 K should
follow reactions:
Fig. 5. Hydrogen desorption curves of Zr2CuH4.4 as function of increasing tempera-
ture. The regions from (I) to (IV) were divided by the sharp change in the pressure.
instead of Zr2CuHx, x = 1.0–1.4. The phase transformations of Zr2Cu by
hydrogen absorption above 973 K are described by
Zr2Cu + H2 → ZrCu + ZrHx + H2(ˇ phase) → Zr7Cu10 + ZrHx
+ H2(ꢀ phase).
(12)
The hydrogen absorbing behavior of Zr2Cu is controlled by
hydrogen-induced disproportionations and the absorbing behav-
ior below 973 K as scheme (8) differs from scheme (12) at more
than 973 K.
3.2. Hydrogen desorption behavior
Fig. 5 shows the hydrogen desorption behavior from Zr2CuH4.4
.
The abscissa and ordinate represent the temperature and the
hydrogen pressure in the closed system, respectively. Since the
hydrogen desorption behavior with ramp rates of 7.4 and 3.7 K/min
essentially agree among each other, the desorption process is con-
sidered to proceed under quasi-equilibrium conditions. Hydrogen
release was only noticed at temperatures above 370 K. At temper-
ature of about 560 K, the pressure increase rate suddenly slowed
down but thereafter increased lineally up to about 900 K. At about
930 K, the pressure sharply rose and the curve showed an inflection
at about 950 K. A further sharp pressure rise was observed at about
1020 K followed by an additional inflection of the pressure curve.
In view of the sharp pressure changes observed experimentally at
temperatures of about 520, 930 and 1020 K, the desorption can be
subdivided into four regions, which is designated by (I), (II), (III)
and (IV) (see Fig. 5). A similar type of pressure curve has also been
observed for Zr2M (M = Fe, Co, Ni) hydride [1]. According to hydro-
gen release behavior measurements from Zr2CuH4.2 observed in
thermogravimetric (TGA) and differential thermal analysis (DTA)
observed by Bowman Jr. et al. [8], an exothermic peak appears at
550 K, and two endothermic peaks are observed around 950 and
1000 K. These peaks are in line with the boundaries in Fig. 5. To con-
firm the phase transformation during hydrogen desorption, XRD
measurements were carried out at various temperatures in accor-
dance with the hydrogen desorption experiment. Fig. 6 shows the
obtained XRD patterns. Although the XRD pattern for 600 K was
obscure, the peak of Zr hydride [10] was identifiable. This consti-
Zr2Cu + 0.775H2 → ZrCu + ZrH1.55
and/or
(9)
5Zr2Cu + 1.55H2 → Zr8Cu5 + 2ZrH1.55
,
(10)
where the composition of the Zr hydride was estimated from Ref.
[13]. The hydrogen content by reaction (9) is [H]/[Zr2Cu] = 1.55. This
value is close to the result of the hydrogen absorption at 1023 K
(see Fig. 2). On the other hand, the hydrogen content for reaction
(10) is [H]/[Zr2Cu] = 0.62, a value that disagrees with the result at
1023 K. Consequently, the hydrogen absorption at 1023 K should
be expressed by reaction (9). However, the resultant ZrCu should
decompose during cooling down to ambient temperature owing to
the eutectoid reaction:
ZrCu → (Zr8Cu5 + Zr7Cu10) → Zr2Cu + Zr7Cu10
.
(11)
Consequently, the pattern (d) in Fig. 3 shows the peaks cor-
responding to Zr2Cu, Zr7Cu10 and Zr hydride. The present results
indicate that the ␤ phase should consist of ZrCu and Zr hydride

M. Hara et al. / Journal of Alloys and Compounds 487 (2009) 489–493
493
Zr2Cu has reproportionated. This recovery process is discussed
below. The products formed after hydrogen desorption up to 980 K
were found to be Zr7Cu10, Zr2Cu and Zr hydride as shown in
Fig. 6(b). These products were generated from the products of reac-
tion (13). The relative peak intensity of Zr7Cu10 decreased with
increasing temperature and the peaks of Zr hydride disappeared at
1050 K and above as shown in Fig. 6. Thus, disproportionated Zr2Cu
has reacted to yield the starting material when heated at tempera-
tures above 980 K. Therefore, hydrogen desorption from Zr2CuH4.4
takes place by
Zr2CuH4.4 → unknown + ZrHx + H2 → Zr7Cu10 + ZrHy
+ H2(ꢀ phase) → Zr2Cu + 2.2H2.
(14)
4. Conclusions
In this investigation on the hydrogen absorption and desorp-
tion behavior of Zr2Cu, it was found that Zr2CuH4.4 is formed under
moderate hydrogen absorption at ambient temperature and a
hydrogen-induced disproportionation of Zr2Cu takes place already
at temperatures of about 550 K. The results confirm that Zr2Cu
hydride is a metastable phase.
The unknown compound in the ␥ phase can be assigned to
Zr7Cu10. The ␤ phase reported by Kadel and Weiss consists of
ZrCu and Zr hydride instead of Zr2CuHx, x = 1.0–1.4. The dispro-
portionation behavior above 973 K differed from that below
973 K due to the existence of ZrCu intermetallic compound. The
disproportionation processes below 973 K follows the reaction
Zr2Cu + H2 → Zr7Cu10 + ZrHx + H2(␥phase) → Zr14Cu51 + ZrHx + H2(␦
phase), and that above 973 K the reaction, Zr2Cu + H2 →
ZrCu + ZrHx + H2(␤ phase) → Zr7Cu10 + ZrHx + H2(␦ phase).
Fig. 6. (a–d) X-ray diffraction patterns of Zr2Cu specimens after hydrogen desorp-
tion experiments. Each specimen was prepared by heating up to a given temperature
in the close system, from which the released hydrogen was pumped off. In addition,
the specimen was rapidly cooled to ambient temperature.
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tutes direct evidence for the occurrence of disproportionation. The
initiation temperature of the disproportionation can be obtained
from the hydrogen desorption curve of Zr2CuH4.4. The boundary
temperature of region (I)–(II) agreed with the exothermic peak at
550 K of the DTA reported by Bowman Jr. et al. [8]. Since the decom-
position of the hydride is generally endothermic, the exothermic
peak at 550 K is assigned to the disproportionation. Therefore, the
disproportionation of Zr2Cu occurs at about 550 K. The hydrogen
desorption reaction at the boundary of region (I)–(II) is considered
to be
[11] JCPDS-ICDD, PDF 42-1185.
[12] JCPDS-ICDD, PDF 43-0993.
[13] W. Wang, D.R. Olander, J. Am. Ceram. Soc. 78 (1995) 3323–3328.
[14] JCPDS-ICDD, PDF 47-0983.
Zr2CuH4.4 → unknown compound + ZrHx + yH2.
(13)
On the other hand, the diffraction peak of Zr2Cu appeared at
temperatures above 980 K, indicating that the disproportionated